1. Field of the Invention
The present invention relates to a multi-piece solid golf ball comprising a solid core coated with a polyurethane cover.
2. Description of the Related Art
An ionomer resin cover is primarily used as a cover for a solid golf ball in which a vulcanized rubber sphere is used as a solid core, because of its excellent durability. However, compared to the balata rubber cover, an ionomer resin cover tends to give a far inferior shot feeling to the golfer when hitting the golf ball.
To improve the shot feeling of the ionomer cover, a cover comprising an ionomer mixture has been proposed in Japanese Patent No. 2709950, where a hard ionomer which is a sodium or zinc salt of an olefin-unsaturated carboxylic acid copolymer is mixed with a soft ionomer which is a sodium or zinc salt of an olefin-unsaturated carboxylic acid-unsaturated carboxylic ester terpolymer. The shot feeling can be softened by blending this soft ionomer, but on the other hand, the advantages inherent in the ionomer cover are sacrificed, such as abrasion resistance (chunking properties) and resilience.
Recently, polyurethane has the focus of an inexpensive cover material that imparts to the golfer a shot feeling analogous to that imparted by the balata cover and possessing greater durability than the balata cover. For example, a polyurethane cover comprising a urethane prepolymer which has been hardened with a slow-reacting polyamine curing agent has been disclosed in Japanese Patent No. 2662909. A cover characterized in using a thermoplastic polyurethane elastomer has been proposed in Japanese Unexamined Patent Publication No. H09-215778.
However, there is a problem in the forming of the polyurethane cover. The problem is that it is difficult to form the cover because the rapid reaction between the urethane prepolymer and polyamine curing agent results in a precipitous increase in viscosity. Although the slow-reacting polyamine curing agent is used in Japanese Patent No. 2662909 to prevent the rapid increase in viscosity resulting from the progress of the reaction between the urethane prepolymer and polyamine curing agent, it is still sometimes difficult to form the cover, because of the rapid increases in viscosity, depending on the type of urethane prepolymer, type of curing agent, a combination thereof, and the like. Further improvement is also needed because the resilience, spin performance, and chunking properties are not altogether satisfactory, even when the cover can be formed.
On the other hand, although the use of a thermoplastic polyurethane elastomer results in better formability than the thermosetting polyurethane cover, the lack of three-dimensional cross-linking points results in inferior wear resistance, tear strength, and chunking properties compared to the thermosetting polyurethane cover or the ionomer cover.